Vokera 28compact combi expansion tank problem?

Since putting central heating on in October I have had a problem with pressure dropping and water coming out of the boiler overflow, however I had a burst pipe outside repaired recently and afterwards the boiler made strange sounds and water pressure was strange for a day.In fact hot tap water was a bit iffy.I've been told that it's the expansion tank and 'cos it's a vokera the heat exchanger will be next and it would be cheaper to get a new boiler.It's only 5 years old!